A wearer may be prescribed a positive or negative optical power correction. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition representing the power increment between far vision and near vision. The addition is qualified as prescribed addition. Ophthalmic lenses suitable for presbyopic wearers are multifocal lenses, the most suitable being progressive multifocal lenses.
The ophthalmic prescription can include a prescribed astigmatism. Such a prescription is produced by the ophthalmologist in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). The amplitude value represents the difference between minimal and maximal power in a given direction which enables correction of the visual defect of a wearer. According to the chosen convention, the axis represents the orientation of one of two powers with relation to a reference axis and in the sense of rotation chosen. Usually, the TABO convention is used. In this convention, the reference axis is horizontal and the sense of rotation is counterclockwise for each eye, when looking at the wearer. An axis value of +45° therefore represents an axis oriented obliquely, which when looking at the wearer, extends from the quadrant located up on the right to the quadrant located down on the left. Such an astigmatism prescription is measured on the wearer looking in far vision. The term <<astigmatism>> is used to designate the pair (amplitude, angle); despite this use not being strictly correct, this term is also used to refer to the amplitude of the astigmatism. The person skilled in the art can understand from the context which meaning is to be considered. It is also known for the person skilled in the art that the prescribed power and astigmatism of a wearer are usually called sphere SPH, cylinder CYL and axis. FIG. 1c is a schematic illustration of the prescription expressed in TABO referential desired for the left eye of a wearer. The axis of the prescription (65° here) gives the direction of the smallest power which is, in this case, 3.50 δ whereas the highest power is along the direction which is perpendicular to the axis of the prescription and its value corresponds to +3.50 δ+0.25 δ=3.75 δ. The mean power (also called the mean sphere SM) is the arithmetical average of the smallest power and the highest power and is equal to 3.625 δ.
As explained above, the most suitable lenses for presbyopic wearers are progressive multifocal lenses. However, such lenses induce optical defects that must be minimized in order to satisfy the wearer. When an image perceived by a wearer is formed through a lens, several phenomena degrading the imaging performances of a lens occur. Power defect, astigmatism defect and high order aberrations are examples of optical defects which impact the optical quality of the image, then reducing its sharpness and its contrast. The optical defects also modify the appearance of the object perceived by the wearer. Indeed, an object may appear distorted (the shape of the image is modified) and/or delocalized compared to the actual object.
When designing a progressive multifocal lens, it is therefore sought to reduce as much as possible the optical defects even though it is not possible to cancel them completely because of the power increment. Thus, it is also sought to spread the defects in such a way that the wearer's vision is the least affected by the remaining optical defects.
The person skilled in the art knows how to compensate for optical defects which comprise among others the power defect and astigmatism defect as described in EP 0990939, U.S. Pat. No. 5,270,746 (EP 0461624) and WO/1998/012590. The lens designer has to handle two contradicting constraints when compensating for the optical defects. On the one hand, he needs to design large central zones to provide the wearer with comfortable vision, when reading for instance. This can be done by displacing the optical defects to lateral zones of the vision field thereby producing important gradients in the periphery of the vision field which impact dynamic vision. On the other hand, the designer needs to limit the gradients in the periphery of the vision field to improve dynamic vision; this being detrimental to the size of the central vision zone. Known methods require a compromise between central and peripheral vision performances.
Moreover, the above-mentioned methods only consider optical criteria which first of all improve or degrade the sharpness of the image perceived by the wearer. For instance, criteria of power, astigmatism and higher order of aberration are dealt with. The lens designer will make a compromise among those criteria to limit distortion of the image perceived through the lens. Thereby, the lenses are typically a compromise between sharpness and image deformation.
If the front face surface is spherical in a full back side (FBS) lens, for example, front and back surface misalignment does not result in an optical error. Adding a toric surface to its front surface allows for reduction of the lens distortions. The greater a cylinder value of the toric surface, the higher the lens distortion reduction. However, with such a lens design. if a misalignment exists between the front and back surfaces of the lens as shown in FIG. 1a-2, FIG. 1a-3, and FIG. 1a-4, an unwanted astigmatism is produced on the lens. In particular, at a far vision diopter measurement position (“FV position”) point of a wearer, it is more difficult to meet the ISO standard tolerances regarding prescribed astigmatism (see FIG. 1b).
Many conventional manufacturing laboratories for making ophthalmic lenses use standard equipment that have an alignment accuracy between the front and back surfaces that is not as high as is available with high end equipment. As shown in FIG. 1a-1, the front and back surfaces are aligned when their Z axes coincide and the respective x,y axes are not rotated relative to each other. FIG. 1a-3, FIG. 1a-4 and FIG. 1a-2, respectively, show that misalignment between the two lens surfaces can be due to translation along the X axis, with a value of Tx, translation along the Y axis, with a value of Ty, and/or rotation around the Z axis, with an angle of Rz.
According to applicable manufacturing standards, the finished lens has an astigmatism tolerance of 0.12D. This requirement must be met after all the potential sources of error have been taken into account. Misalignment is just one such potential source of error. In a conventional laboratory for manufacturing progressive lenses, the alignment accuracy is difficult to minimize without significantly modifying the conventional lens finishing process. As a result, yields for final lenses are significantly reduced when using a front toric surface.
As shown in FIG. 1b, for a tore of 1.0D, just from the Rz misalignment error due to the manufacturing process with use of the standard equipment, the astigmatism tolerance of 0.12D can be exceeded. If the tore is reduced to 0.75D, some margin exists to accommodate other potential sources of error, but the margin is quite small and, actually, is insufficient. The margin increases as the tore is further reduced to lower values. However, lower values of tore do not provide adequate lens distortion compensation. Thus, a progressive lens design is required that can accommodate the misalignment tolerances of a standard lab, provides the desired level of distortion compensation, and yet leaves a sufficient margin for other potential sources of error without exceeding the 0.12D permitted tolerance for a finished lens.